Microphone circuit



MICROPHONE CIRCUI T Filed June 2; 1924 w! NESSE INVENTOR (7/0/00 1? flan/7a BY Q 17 W sponding to sound and delivering electric currents corresponding to the sound.

Patented Oct. 16, 1928.

UNITED STATES PATENT. OFFICE.

CLINTON ZR. HANNA, F WILKINSBURG, PENNSYLVANIA, ASSIGNOR TO WESTING- HOUSE ELECTRIC 8n MANUFAGTURDSTG COMPANY, A CORPORATI ON OF PENNSYL- VANIA.

Application filed June 2,

This invention relates to devices for re- It is an object of this invention to provide a device of this class in which the voltage produced shall be proportional to the pressure exerted by the sound received, at substantially all fre uencies.

It is a further 0 ject of this invention to so correlate the electrical constants of the circuit to which a microphone is connected, with the characteristics of the microphone, that the-electrical eilect obtained shall be the same, for the same amount of sound energy received, over a very considerable range of frequency.

By properly designing the correlations just mentioned, it is possible to give any desired character to the variation of the voltage produced with the frequency. The invention, therefore, is not limited to keeping this variation as small as possible, although that is usually the desired result.

Other objects of the invention and details of the construction will be clear from the following description and the accompanying drawin in which the single figure is a diagram il ustrating the circuits and apparatus which embody my invention.

The microphone 1 ispart of a local circuit including an inductor 2, a condenser 3 and asecond condenser 4, all arranged in series.

As will be pointed out below, a portion of the inductance or one of the condensers or both the inductance and a condenser may be inside of the casing of the microphone '1 or may even constitute parts of the microphone itself.

A second or operating circuit, of which the energization is controlled by the conditions I in the local circuit including the microphone, includes a terminal 5, which-adjustably divides the inductor-2,:i'nd a terminal 6 between the two condensers 3 and 4.

I have shown the condensers as adjustable but, in the actual use of the device, these condensers, having once been adjusted, would remain undisturbed.

The inductor 2 and the condensers 3 and 4 are chosen of such values, or are adjusted to such values, that the circuit including them in series is resonant at that frequency at which the microphone 1 delivers the MICROPHONE CIRCUIT.

1924. Serial No. 717,244.

greatest electromotive force in response to a given sound energy. .i

Most microphones respond in alresonant manner to a particular frequency, delivering at that frequency a greater electromotive force than at other frequencies. With a microphone of this character, the current. flowing in the series circuit will be a maximum at that frequency at which the microphone is resonant.

The terminal '5 is adjusted alon-gthe induc-v tor 2 and the condensers 3 and 4 are adjusted until the portion of the series circuit between the terminals 5 and 6 is also resonant to the frequency for which the microphone is resonant. In performing this ad ustment, the equivalent capacity of the two condensers 3 and 4 in series should be left unchanged, but

, in will be a maximum at this particular frequency. The variation of the impedance be tween the terminals 5 and 6 with the frequency is, therefore, in a direction opposite to that of the corresponding variation in the current through said impedance. Conse. quently, the alternating diiference of potential between the points 5 stantially unchanged with changing fre quency. The difference of potential impressed upon the operating circuit, is, there fore, a constant regardless of the changes in frequency in the sound energy, varying only with the amplitude of sound.

The operating circuit may be connected to another potential-responsive device. For the purpose of illustration I have shown it connected to the grid'and filament of a vacuum tube. The output of this tube may be connected to any of the usual radio or other transmitting systems.

It will be obvious that the sharpness of the tuning of the circuit and the sharpness of the tuning of the portion between the terminals 5 and 6 may be varied by distributand 6 remains sub-- ing the resistance throughout the series cir- 01111: in accordance with the desired sharpness of the two tuned connections. By thus varying the relation between the sharpness of each of the two tunings, the curve representing the relation. of the frequency to the electromotive force impressed upon-the operating circuit may be made concave in either direction. It is, therefore, evident that, if the resistance be properly distributed, this curve may be made concave in either direc tion, that is, it may be a straight line.

Because the series circuit through the inductor 5 and the condensers 3 and 4 is resonant,

at the resonance frequency of the microphone,

. the current therein at frequencies below this,

will be leading the electromotive forces while, at frequencies above resonance, it will be lag ing behind the electromotive force. The

lea mg current at frequencies below that of resonance of the diaphra m assists in overcoming thestifi'ness. A agging current at frequencies above the resonance of the diaphragm assists in overcoming the inertia. For these-reasons the resonant character of the series circuit assists in increasing the current delivered by the microphone at nonresonant frequencies.

Atresonance the current is in phase with thevolta'ge and thus acts as an additional load upon the diaphragm. The resonant character of the series circuit, therefore, di-

minishes somewhat the peak of the electro motive force at resonance. These two effects of the resonant character of the series circuit assist in giving a straight line form to the curve, representing the relation of the electromotive force impressed upon the operating circuit to the sound frequency. They may even cause it to be concave, so that a smaller electromotive force is impressed upon the sible to have the terminals 5 and 6 be the terminals of the microphone. The diagram is not to be taken as indicating that the parts 2 to 6 must be outside of the microphone case. I

Other variations of the construction will be apparent to those skilled in the art and the specific form illustrated and described is not to be construed as a limitation. No limitation. is intended except that required.

by the prior artand indicated in the accompanying claims.

I claim as my invention:

1. In a sound-translating device, a microphone having a resonance period, a circuit including said microphone and having the same resonance period, connections to said circuit at separatedpoints, the portion of said circuit between said points also having the same resonance period.

2. In a sound-translating device, a microphone having a resonance period, a circuit including an inductive reactance and 2. capacitative reactance so proportioned that the circuit is resonant at the same periodas the microphone, connection points in said circuit separatin each of said reactances into two parts suc1 that the part of the inductance 4 and the part of the capacity included between said connections points render the portion of the circuit between said points resonant at the'said period.

3. In combination, a micro resonant circuit upon which t e microphone impresses ele'ctromotive force and a circuit connected at two separate points to said series resonant circuit, the electrical dimensions of the portion of the series circuit between said terminals being so correlated to the electrical dimensions of the remainder of said series resonant circuit that the dependence of the electromotive force impressed upon the second-named circuit upon the frequency shall be of any predetermined character.

4. In-a sound translating system, a microphone having a predominant. resonant frequency, a circuit including said microphone and being electrically tuned to said resonant frequency, a. voltage responsive device, and connections between said device and said circuit, that portion of said circuit included between the points of connection of said voltage responsive device thereto being series-resonant to said predominant frequency, whereby when said microphone is energized by sound the potentials across said series resonant por-' tion are substantially independent of the frequency of the sound input to said micro phone.

In testimony whereof, I have hereunto subbone, a series scribed my name this 22nd day of May, 1-924.

' CLINTON R. HANNA. 

